Phages, plasmids, and related genetic elements are being examined in Streptococcus pneumoniae (pneumococcus) as tools for dissecting genetic processes in this and related species. Particular emphasis is currently going to analysis of the nature, organization, and transfer mechanisms of genes responsible for multiple drug resistance in clinical isolates. Several such genes are present as contiguous insertions of DNA, probably of plasmid origin, in the chromosomes of these strains. Conjugal transfer of these genes has been found (the first case of chromosomal conjugation in gram positive bacteria), and it is being analyzed to see how many strains and species show it and for the mechanisms by which it happens. Both inter- and intra-species conjugation has been observed. Plasmids as such are not involved in the above phenomena but can be introduced into pneumococcus. Their transfer by both transformation and conjugation is being examined, and transduction will be examined also. Both temperate and lytic phages are being used as tools for several aspects of these analyses.